That means they don’t have a paid job and haven’t been actively looking for one.

This figure does not include those in jail or prison. It does include students and men staying home to take care of children or other family members — but, as Nicholas Eberstadt estimates in his important new book, “Men Without Work,” these two categories seem to account for less than 15 percent of what he calls the NILFs (for not in labor force). And the NILF share of the U.S. prime-age male population has been growing and growing.

Why? The usual suspects like technology, trade and a lack of motivation (“immigrants and married men are under-represented in the NILF ranks”) are listed along with the novel suggestion that value of leisure time has increased. “The percentage of NILFs has risen since the 1970s all over the developed world, which definitely fits with the technology-displacing-jobs explanation. But the trajectory has been much steeper in the U.S. than in other rich countries.” Why? In short, prison time:

[T]he great incarceration wave that began in the 1970s has produced millions of ex-convicts who are ill-prepared for jobs or are discriminated against by employers even when they are prepared. Eberstadt cites an unpublished study that estimates that 12 percent of the adult male civilian non-institutional population (that is, men not in jail) in the U.S. has been convicted of a felony, and figures the percentage must be even higher for prime-age men given that the “incarceration explosion” didn’t start till the 1970s.

This is on the one hand tragic: Millions of American men who were imprisoned in the 1970s through 1990s have been thrust into a labor market that really doesn’t want them. On the other hand, it is at least potentially fixable. Job displacement by technology is probably unstoppable, but how we punish crime is a public-policy choice. Incarceration rates have already been falling with the big declines in crime since the early 1990s, and the past few years have seen the growth of a bipartisan consensus (interrupted by the current presidential campaign, to be sure) that the U.S. throws too many people in prison for too long and doesn’t do nearly enough to rehabilitate them. Prison and sentencing reform might actually be the country’s best shot at thwarting that “linear trend” that would put a quarter of prime-age men out of work by 2050.